Activins and Bone Morphogenetic Proteins (BMPs) are members of the much larger Transforming Growth Factor-beta (TGF-β) superfamily. Due to their pervasiveness in numerous developmental and cellular processes, TGF-β ligands have been the focus of great interest. For TGF-β ligands to be successfully used as therapeutic tools, several hurdles need to be overcome. The ability to specifically modify and alter the properties of TGF-β ligands, as well as generate those ligands in significant quantities is required.